1. Field of the Invention
The present invention relates to a data communication apparatus having a full duplex communication function.
2. Related Background Art
An image communication apparatus having an image transmission function according to error control protocols is known as a conventional data communication apparatus of this type. An apparatus for performing half duplex communication in pre- and post-message procedures and full duplex communication in only image transmission is known as described in "Shisetsu" (Vol. 38, No. 5, 1986, P. 59) published by NTT.
A receiver for such a communication apparatus acknowledges to a sender that the receiver has an error control function by using initial identification signals NSF (nonstandard apparatus) and DIS (digital identification signal). In response to these signals, the sender sends back receive command signals NSS (nonstandard apparatus setting) and DCS (digital command signal). This communications allows the sender to acknowledge that the error control protocols are effected. The sender then sends out a training check signal TCF to the receiver. The training check signal is a signal of all "0"s and is transmitted for 1.5 seconds at an image signal bit rate of 9,600 bps, 7,200 bps, 4,800 bps, or 2,400 bps.
The receiver detects a receive-ready signal CFR sent from the receiver in response to the training check signal TCF. The sender starts transmitting the image signal.
When a receive ready acknowledge signal CFR is received from the receiver in response to the training check signal TCF, the sender starts transmission of the image signal.
The receiver demodulates the received image signal and performs error monitoring. When an error is detected, the receiver transmits an error acknowledgment signal NACK.
The sender monitors reception of the error acknowledgment signal NACK during transmission of the image signal. When the sender detects the error acknowledgment signal, the sender stops transmitting the image signal. The sender receives a control signal from the receiver after the sender has received the error acknowledgment signal NACK. In response to this control signal, the sender starts retransmitting the image signal from a designated portion.
In this manner, high-quality facsimile transmission free from errors can be performed.
However, the conventional visual communication system described above has the following disadvantages:
(1) A communication circuit state in a direction from the sender to the receiver may be different to that in a direction from the receiver to the sender. For this reason, even if the image signal is normally transmitted from the sender to the receiver, the error acknowledgment signal may not be accurately detected by the sender upon detection of a reception error. A normal error control protocol cannot then be effected between the sender and the receiver. As a result, a communication disable state is caused to result in a disadvantage (the first disadvantage).
(2) A communication defect may occur due to transmission delay of the transmission line. More specifically, even if the receiver detects an error of the image signal and sends out the error acknowledgment signal NACK, it takes at least a delay time of, e.g., about one second, to send the error acknowledgment signal NACK to the sender. In this case, in order to retransmit data, the sender must store data, the volume of which corresponds to 2 seconds. For example, if a transmission speed of the image data is 9,600 bps, a memory capacity of 2.4 kbytes (=(9,600.times.2)/8) is required. In addition, when detection time of the error acknowledgment signal NACK and an influence of an echo suppressor in international communications are taken into consideration, a larger memory capacity is required.
When a facsimile system having a small memory capacity is used to perform communication through a transmission line having a large transmission delay, an image cannot be transmitted again, and communication cannot be established, thus causing a disadvantage (the second disadvantage).
(3) In order to prevent the first and second disadvantages, a communication apparatus may have a memory having a large capacity. However, this results in a bulky apparatus at high cost, thus causing a disadvantage (the third disadvantage).
Other data communication apparatuses are described in U.S. Ser. No. 679,558 (Dec. 7, 1984), U.S. Ser. No. 834,779 (Feb. 28, 1986), and U.S. Ser. No. 847,684 (Apr. 3, 1986) filed by the present applicant. The apparatus described in U.S. Ser. No. 679,558 aims at performing error rate check more strictly as time passes. The apparatus described in U.S. Ser. No. 834,779 aims at retransmitting data when the predetermined number of successive errors is detected. The apparatus described in U.S. Ser. No. 847,684 aims at retransmitting data by causing a sender and a receiver to control a function of an echo suppressor.